1. Field of the Invention
This invention relates to a SAW resonator filter and more specifically, the present invention relates to a SAW resonator filter having a narrow bandwidth and having a compact size.
2. Description of the Related Art
A SAW (surface acoustic wave) filter is widely used as a bandpass filter in communications devices and other electronic devices. SAW filters include a transverse-type SAW filter, which has two interdigital transducers (IDTs) arranged on a piezoelectric substrate with a predetermined distance therebetween, and a SAW resonator filter having a resonator including an IDT provided on a piezoelectric substrate.
An edge reflection type SAW resonator filter using Shear Horizontal (SH) surface acoustic waves such as Love waves and Bleustein-Gulyaev-Shimuzu (BGS) waves and other similar waves, is known as a SAW resonator filter. In an edge reflection type SAW resonator filter, the resonator is provided between two opposite edges of the piezoelectric substrate, and the edges are used to reflect the SH waves. Since reflectors are not needed, a compact filter can be realized.
Further, an edge reflection type longitudinally coupled SAW resonator filter, and a transversely coupled SAW resonator filter, which are made by coupling two SAW resonators, is already known (e.g. Japan Unexamined Patent Publication Nos. 9-69751 and 10-261938).
FIG. 19 is a perspective view showing an example of a conventional edge reflection type, longitudinally coupled SAW resonator filter. The longitudinally coupled SAW resonator filter 51 includes IDTs 53 and 54 which are provided on a rectangular piezoelectric substrate 52. The IDTs 53 and 54 are edge reflection type surface acoustic wave resonators, and are aligned along the direction of propagation of the surface acoustic wave.
Alternatively, FIG. 20 shows a conventional edge reflection type, transversely coupled SAW resonator filter. The transversely coupled SAW resonator filter 61 includes IDTs 63 and 64 which are provided on a rectangular piezoelectric substrate 62. Each of the IDTs 63 and 64 is an edge reflection type surface acoustic wave resonator, but here the IDTs 63 and 64 are aligned perpendicular to the direction of propagation of the surface acoustic wave.
There is a great demand for a filter having a narrow bandwidth in order to improve the degree of selection. In the conventional SAW resonator filters 51 and 61, the bandwidth is reduced by methods that include: (1) using piezoelectric substrates 52 and 62 that have small electromechanical coupling coefficients; (2) making the thickness of the electrodes of the IDTs smaller, thereby lowering the effective electromechanical coupling coefficients thereof; or (3) adjusting the distance between the IDTs, and bringing the resonant frequencies of the two resonators closer together; and other similar methods.
The problem with the first method is that it is necessary to select a piezoelectric material for the piezoelectric substrate that is suitable for its intended purpose, and it is difficult to obtain a material which has an electromechanical coupling coefficient appropriate for the required bandwidth. Further, it is extremely difficult to find materials that are suitable for the necessary bandwidth and which also have excellent temperature characteristics
The problem with the second method is that when the thickness of the electrodes of the IDTs is decreased, vibration energy caused by the piezoelectric effect leads to conversion of the waves to bulk waves and deterioration the desired filter characteristics. Further, there are limits to the amount of narrowing of the bandwidth that can be achieved.
The problem with the third method is that when the resonant frequencies of the two resonance modes are brought too close together, the two resonance modes become almost joined, thus increasing insertion loss.
To overcome the problems described above, preferred embodiments of the present invention provide a SAW resonator filter having a narrow bandwidth without increasing the insertion loss, having excellent temperature characteristics, and IDTs that have sufficient electrode thickness.
A preferred embodiment of the present invention provides a SAW resonator filter which generates SH-type surface acoustic waves and that includes a piezoelectric substrate, and first and second resonators arranged on the piezoelectric substrate, the first and second resonators including first and second interdigital transducers having electrode fingers, respectively, wherein the first and second interdigital transducers are acoustically coupled to form a filter, and wherein the first and second interdigital transducers are divided into a plurality of sub-interdigital transducer portions, respectively.
In one preferred embodiment of the present invention, the first and second IDTs are preferably divided to have two to four sub-IDT portions.
Also, in other preferred embodiments, the first and second interdigital transducers may either be longitudinally coupled or transversely coupled.
Note that by dividing the interdigital transducers to have multiple sub-IDT portions, the effective electromechanical coupling coefficient is reduced, thereby enabling the bandwidth to be made narrower. Further, the improvement in electrode structure allows for the filter to use a piezoelectric substrate having excellent temperature characteristics, so that it is possible to achieve a SAW resonator filter having a narrow bandwidth and superior temperature characteristics. Moreover, when an edge reflection type SAW resonator filter using SH-type surface acoustic waves is made, it is possible to provide a compact band filter having low loss and excellent selectability.
In another preferred embodiment of the present invention, a communication apparatus includes a duplexer, wherein the duplexer includes the above-described resonator filter having first and second interdigital transducers that are divided into a plurality of sub-interdigital transducers, respectively.
Other elements, features and advantages of the present invention will be described in detail below with reference to preferred embodiments of the present invention and the attached drawings.